This question demonstrates some more detailed hydrologic connections via the aquifers. First we identify private wells that tested above 200 ng/l in PFOA. Next we determine which aquifers they are likely fed by based on proximity in the same S2 cell. Then we discover all other private water supply wells that are likely fed by the same aquifer.
This question facilitates prioritization of private well water testing by identifying high risk wells that have not been tested based on the existing data. It provides a more sophisticated hydrological modeling than simple proximity.
The code for this example is available here .